Fluorescence in situ hybridization (FISH) is being used increasingly in clinical practice; however, current FISH techniques require fresh material, and there is considerable variation in hybridization efficiency between laboratories. We have modified a FISH technique described by Pinkel et al. (1986) that works not only on freshly G-banded material but also on cytogenetic preparations ranging in age from 2 wk to 12yr. We have tested this technique on several centromeric alphoid satellite probes (D1Z5, D7Z1, D17Z1, DXZ1, and DYZ3) and one noncentromeric minisatellite probe (D1Z2). Our average hybridization efficiency on freshly banded preparations for these probes is consistently greater than 90 %. The combination of higher efficiency and the ability to perform hybridization on previously G-banded material makes this a valuable technique for retrospective analyses.